1. Field of the Invention
This invention relates generally to welding of ductile iron work pieces for weld securement in pressure, stress, and abuse critical locations, as well as lighter duty sites. More particularly, the invention is directed toward a weld joint combining both relatively high ductility and excellent soundness.
2. Description of Related Art
Numerous welding processes are known in the art, including Metal-Inert-Gas (xe2x80x9cMIGxe2x80x9d) welding, which employs a metal electrode or filler metal and a shielding gas to enhance weldability. Although not the oldest form of welding, MIG welding is an old art, most basically described in U.S. Pat. No. 1,589,017, issued to Lincoln, which takes specific resort to alloyed filler metals, such as nickel-steel or manganese-steel. In typical welding of structures by MIG welding, a first welding pass is made, which both liquefies the upper surface of the base metal of the workpieces being welded, and deposits a portion of a consumable electrode (a xe2x80x9cweld wirexe2x80x9d) at the intersection, or xe2x80x9crootxe2x80x9d of the workpieces. The liquefied base metal and deposited weld wire form a weld puddle when in the molten state, which solidifies into the weld body itself, to this point the single solidified pass is the entire weld body. Because a single pass of this welding procedure may not provide the desired holding power of a weld for the particular purposes chosen, artisans frequently build up the weld with multiple overlapping passes.
As each subsequent weld pass builds on the previously laid weld body, a portion of the area of the weld body deposited by the immediately previous weld pass is similarly liquefied, and brought into molten state amalgam with the newly deposited weld wire. By this procedure, a weld body is built up in depth from the root by a series of passes, with lower order (e.g., earlier welded) passes being overlaid and interfused with higher order (later welded) passes until the weld exhibits a depth and breadth determined to be sufficient for the characteristics of the weld required. Under certain conditions the variously ordered passes can be visually identified with the aided or even the naked eye in a cross-sectional segment of the weld body. Whether visually identifiable or not, the resultant weld body possesses multiple solidified passes, areas attributable to lower ordered and higher ordered passes, in succession. Frequently, the precise composition of each order pass will differ from the next higher and the next lower pass because of different weld wire selection used for the respective passes, or, even where the weld wire is not altered, because of dilution effects occurring from lower order pass to higher order pass.
Welding ductile iron work pieces for securement to other ductile iron work pieces by the MIG and other methods poses such difficulties that the fabrication of pipes and structures by such ductile iron-to-ductile iron welding is relatively rare. Spot and repair welding of ductile iron structures does occur of necessity in the field, but in the past the process has been problematic enough to discourage use of the ductile-to-ductile welding for purposes of securing joints or plates where failure of the weld would amount to catastrophic failure of the work piece (e.g., at the intersection of perpendicular pipe lengths).
Welds securing an in-line radial connection (i.e., a xe2x80x9cTxe2x80x9d intersection) between pipes may be subjected to sudden impacts or high bending moments, which are unpredictable due to the environments in which the pipes are laid and to the unanticipated installation events that may from time to time occur. Under such conditions, ajuncture created by a brittle weld will tend to fail, while a resilient weld will accommodate such forces without cracking. The difficulty of obtaining a resilient ductile iron to ductile iron weld is exacerbated by the fact that the heat produced during the welding process tends to create an area around thejoint known as a xe2x80x9cheat affected zonexe2x80x9d (a xe2x80x9cHAZxe2x80x9d), in which the ductility of the ductile iron is adversely affected by the heat. In general, the HAZ exhibits an increased brittleness. The inventors note that because of the brittle nature of the HAZ, the welded juncture in the prior art tends to fail even where the weld metal deposit itself does not fail, with the result that cracking or breaching may be observed in the HAZ surrounding the weld. Because of the difficulties posed by the HAZ, the art has focused on solving the ductile weld difficulties in two manners. First, artisans typically attempt to reduce the size of the HAZ. This means introducing the least amount of heat possible to the joint, which necessitates a focus on smaller welds that exhibit a firm bond at low order passes. The second known solution entails extending the weld a great distance from the root/throat, to either distribute the HAZ over a greater area or to move the HAZ to a region of lower stress.
Where welding is performed on ductile iron, weld wires, which may be filler metal or consumable electrode, frequently include an alloy of nickel. Typical nickel alloy weld wires include 44% or 55% nickel, although other alloys may be available in varying percentages. Higher purity nickel weld wire is understood in the art to be primarily useful only for single, or at most double, pass welding situations; typically this high purity nickel is used only in spot repair. It is known in the art that when a weld necessitates building up by multiple passes, a higher order pass using a weld wire with high nickel purity tends to exhibit undesirable cosmetic characteristics such as tension anomalies (those in the art occasionally refer to such tension anomalies variously as xe2x80x9cinclusions,xe2x80x9d xe2x80x9cporosity,xe2x80x9d xe2x80x9cpitting,xe2x80x9d and other terms indicating a perceptible lowering of smoothness or purity of the weld body, particularly at the surface, but also in cross-sectional view). Because of difficulties in achieving a xe2x80x9csoundxe2x80x9d weld (a weld that substantially avoids or reduces tension anomalies), higher order passes are believed by the art to require either an alloy containing less than 86% Nickel, or a flux core or coated weld wire that (1) dilutes the nickel content in the weld puddle and (2) may produce a slag or other weld puddle surfactant. This focus by the art is exemplified by U.S. Pat. No. 3,328,557, issued to Rogers, which addresses the difficulties and proposes a tubular nickel electrode having a flux core; by U.S. Pat. No. 3,301,997, issued to Semenchuk, and by U.S. Pat. No. 2,900,490, issued to Petryck, which each teach that when using a xe2x80x9chigh nickelxe2x80x9d content wire base, the weld puddle must be diluted with non-nickel additives or alloys.
Due in part to issues such as those discussed above, the art considers a sound and cosmetically clean higher order pass of undiluted nickel weld wire to be beyond reach. Suppliers of high purity nickel weld wire have informed the inventors that it is prohibitively difficult to produce a sound and cosmetically clean weld at weld passes higher than second order passes. Present product literature regarding high nickel content welding wire suggests use of such wire for only first order (single pass) welds or second order (dual pass) welds that will be machined, as is exemplified by Inco Alloys International""s product sheet for NI-ROD Filler Metal 99. As described in the claims and illustrated in the summary and detailed description below, the inventors disclose a weld having both of these characteristics of soundness and ductility at higher order passes.
The art has experienced difficulty obtaining xe2x80x9csoundxe2x80x9d welds with high-nickel welds above the second pass. Particular problems are tension anomalies and sub-optimal interweld pass fusion at higher order passes of the weld wire, along with undesirably high heat levels transferred to the workpiece. The inventors have discovered that ductile iron-to-ductile iron welds can be reliably formed at mission-critical junctures by compensating for the brittle nature of the heat affected zone through enhancing ductility of the weld itself. The present invention provides in continuing part for a highly durable weld without soundness problems such as tension anomalies, multilayer fusion imperfections, or pinholes; and a method of forming such a weld. The weld is created using high nickel weld wire (filler metal or consumable electrode, or both) welded in a pure inert gas (or mixture of pure inert gasses and even reducing gasses). Additions of weld metal alloys, flux coring, and deoxidizing coatings are not required. The work pieces do not require pre- or post-heat treatment to minimize the effects of the HAZ. Additional inventive features are described more fully below.
The following stated objects of the invention are alternative and exemplary objects only, and no one or any should be read as required for the practice of the invention, or as an exhaustive listing of objects accomplished.
As suggested by the foregoing discussion, an exemplary and non-exclusive alternative object of this invention is to provide a method of producing a sound weld between work pieces, at least one of which is ductile iron.
A further exemplary and non-exclusive alternative object is to provide a reliable and ductile weld between work pieces, at least one of which is ductile iron, the weld being substantially pure nickel at those areas of the weld deposited by higher order passes than the second order pass.
A still further exemplary and non-exclusive alternative object of the invention is to provide a method of providing a ductile weld that substantially eliminates the need for physically removing slag and scale from the weld bead between passes.
A still further exemplary and non-exclusive alternative object of the invention is to provide a method of welding that exhibits enhanced weldability at higher order passes.
The above objects and advantages are neither exhaustive nor individually or collectively critical to the spirit and practice of the invention. Other or alternative objects and advantages of the present invention will become apparent to those skilled in the art from the following description of the invention.